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JEE Advanced levelSingle correct optionDifficulty 3 of 3g2215

Acetic acid has pKa 4.76 in water and 12.3 in DMSO. By what factor is $K_{a}$ in water larger than $K_{a}$ in DMSO?

Aabout $2.6$
Babout $3.5 \times 1 0^{7}$
Cabout $3.5 \times 1 0^{- 8}$
Dabout $7.5$

Answer: B

K_{a} (water) / K_{a} (DMSO) = 1 0^{12.3 - 4.76} = 1 0^{7.54} = 3.5 \times 1 0^{7}

. The ratio of the pKa values (

12.3/4.76 = 2.6

) is a common error. The enormous difference arises because DMSO cannot hydrogen-bond to acetate.

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